The goal of this program is an analysis of central neural mechanisms participating in diurnal rhythm regulation. A large number of diurnal rhythms have been identified in mammals, including man, and many of these appear to be rhythmic functions generated by genetically-determined central neural oscillating mechanisms. The precise timing of the rhythms, however, is maintained by exogenous entraining stimuli, particularly the diurnal cycle of light and dark. Thus, there are two principal problems in the analysis of these central neural mechanisms. The first is to determine the components of the visual system which mediate the effects of light as an entraining stimulus. In prior studies a direct retinohypothalamic projection was demonstrated to the suprachiasmatic nucleus of the hypothalamus, and this projection can maintain entrainment of diurnal rhythms in the absence of other central retinal projections. The proposed studies will determine whether other visual pathways participate in mediating the synchronizing effects of light using selected lesions in the visual pathway and analysis of activity and drinking rhythm entrainment following changes in the light and dark cycle. The second problem to be analyzed is the localization and organization of rhythm-producing mechanisms. Earlier work has shown that destruction of the suprachiasmatic region abolishes diurnal rhythms in adrenal corticosterone, pineal N-acetyltransferase, activity and drinking. Further studies will analyze the effects of discrete lesions on these rhythms in order to define the exact locus necessary for the maintenance of circadian functions and long term studies will determine if recovery of function occurs or can be induced. Other studies will analyze the effects of suprachiasmatic lesions in the newborn rat to determine if there is greater plasticity in the developing nervous system. Morphological studies will investigate the connections of hypothalamic regions participating in diurnal rhythm regulation using the autoradiographic tracing method, the horseradish peroxidase-retrograde transport method, the Golgi method and electron microscpy. The objective of these studies is to provide a functional map of central mechanisms which participate in diurnal rhythm regulation.